1. Field of the Invention
The present invention relates, in general, to an image forming apparatus. In particular, the invention relates to an image forming apparatus which forms a visible image responding to an image information using electrostatic photography technology.
2. Description of the Related Art
In a conventional image forming apparatus which forms an image using electrostatic photography technology, the image information recording surface of a document is scanned by a light beam and the light reflected from the image information recording surface is focused on the outer surface of an image carrier, e.g., a photosensitive drum, to form an electrostatic latent image on the image carrier. The electrostatic latent image formed on the surface of the image carrier is developed by the electrostatic adhesion of a developing agent to be a visible image.
In general, one of two different developing agents is used to develop the electrostatic latent image formed on the image carrier. One developing agent may be a two component developing agent in which thermoplastic colored toner particles are mixed with carrier particles such as ferromagnetic particles. The other may be a single component developing agent in which each toner particle is integrated with a carrier particle.
The two component developing agent has a good environment-resisting ability, e.g., a high image density against high temperature or high humidity. This is because a high electrostatic charge can be produced by the mechanical friction between the toner particles and the carrier particles in spite of high humidity. On the other hand, the one component developing agent has good maintenance ability during the operational life of the image forming apparatus because of the integration of the toner particle and the carrier particle.
However, if the two component developing agent is used, it is necessary to maintain the mixture ratio between the toner particles and the carrier particles (toner density) at a constant value to produce images having a constant image density. Thus, an auto-toner supply system is employed if the two component developing agent is used.
In the auto-toner supply system, a toner sensor is used to detect the toner density of the two component developing agent stored in the developing device. When a low toner density is detected by the toner sensor, the toner is replenished into the developing device.
The toner sensor is disposed in the developing device. When a toner sensing unit of toner sensor contacts the two component developing agent, the toner sensing unit generates an output voltage corresponding to the toner density (toner/carrier ratio) of the two component developing agent.
In the conventional image forming apparatus employing the above-described auto-toner supply system, the toner of the two component developing agent is given a prescribed quantity of electric charge against the electrostatic latent image formed on the surface of the image carrier. The toner of the two component developing agent is attached to the electrostatic latent image on the image carrier, as a toner image, when the electrostatic latent image is developed, and the toner image is transferred to an image forming medium, e.g., a paper, in the transferring process. Thus, the toner in the two component developing agent is consumed every time the developing precess is carried out.
To decrease the consumption of the toner described above, the toner which remains on the image carrier after the transferring process is completed is collected for return to the developing device. Such a toner collecting operation is called a toner recycling system.
In the above-described toner recycling system of the conventional image forming apparatus, if an image having a low image forming rate (area of images formed on the image forming medium/printable area of the image forming medium) is repeatedly printed or copied, untransferred toner particles on the image carrier are collected for the developing device and the collected toner particles are reused repeatedly. If envelopes are used as an image forming medium to be printed by the above-described image forming apparatus, the image forming rate of each envelope is low, e.g., 1.about.2% (whereas the image forming rate of a paper document is about 5%). This is because, in general, only an address is printed on the envelope. Thus, the above-described toner particles are repeatedly collected and stirred in the developing device, resulting in fine toner particles. Since fine toner particles are gathered to make a mass or group, the volume thereof is increased, although the mass thereof is the same as that in the original state thereof as a whole. Thus, the amount of the two component developing agent which contacts the toner sensing unit is increased and this causes increase in the output of the toner sensing unit. The increase in the output of the toner sensing unit falsely indicates the same result as the decrease of the toner density of the two component developing agent stored in the developing device. Responding to this false result, unused toner is replenished to the developing device and the actual toner density in the developing device becomes excessively high. The background of the image formed on the image forming medium, e.g., a paper, tends to be black, and thus, a fogged image may be formed on the image forming medium.
As described above, if a document having a low image forming rate, such as, e.g., an envelope, is continuously copied by the conventional image forming apparatus, an average toner particle size in the developing device tends to decrease. A particle size of the toner having a diameter of about 11 .mu.m at an initial stage is decreased up to 9 .mu.m (about 20% decrease).
After 15,000 envelopes are copied, the volume of the toner in the developing device is increased in contrast with the decrease in an average particle size of the toner, and thus, the output voltage of the toner sensing unit is increased and unused toner is returned to the developing device. As a result, a fogged image may be formed on the envelope.
In the conventional image forming apparatus, the operational life of a process cartridge including the image carrier (photosensitive drum) and the developing device is determined based on the number of printed or copied image forming mediums. Thus, the process cartridge is not changed to a new one until the number of printed or copied image forming mediums achieves a prescribed value, even if the average particle size of the toner in the developing device has been small.